The six step brushless DC permanent magnet motor (typically controlled by a controller acting according to a commutation truth table) is known as the type of motor which has the highest torque and power density capabilities. Therefore, this motor is becoming increasingly popular in industrial drive applications. However, because of the natural characteristic of the permanent magnets, the magnetic field in the air gap of the motor is constant. The maximum speed of the motor is limited by the supply voltage from its power source (such as a group of batteries, a power supply, or a generator). In other words, when the back EMF of the motor approaches the supply voltage for the motor controller, the controller is not able to maintain or increase motor currents so as to raise the motor speed. This maximum speed is called "base speed". This characteristic results in two problems. The first is that the maximum operating speed of the motor is capped by the supply voltage. The second is that the speed range of the brushless DC motor is limited. To increase the speed range, one has to reduce the voltage sensitivity of the motor so that the back EMF of the motor will match the supply voltage at higher speed. It is well known by the brushless DC motor industry that the torque sensitivity is proportional to the voltage sensitivity. Lower values of the voltage sensitivity result in higher motor currents. Therefore, one has to increase the capacity of power electronic devices, such as MOSFETs or IGBTs, in the motor controller, to overcome these problems. The cost and size of the motor increase correspondingly. These penalties have restricted expanded use of brushless DC motors in the industrial drive business.
The primary motivation for the present invention is to solve the above mentioned problems, i.e., to widen the motor speed range without cost and size penalties. The invention provides a reliable method for closing the motor speed loop within the motor's entire speed range, i.e., from zero speed to speeds far above (e.g. even 200-300% above, e.g. at least about 100% above) the base speed for any surface mounted brushless DC motor with two or more winding phases, with a set of normal Hall position sensors for commutation (which are conventionally used in brushless DC motors). The term "surface mounted" means that the magnets are placed on the surface of the motor rotor.
In attempting to solve these problems previous efforts have focused on sine current wave form brushless permanent magnet motors with three current sensors plus much more sophisticated commutation means, such as resolvers or encoders. Representative U.S. patents illustrating this are U.S. Pat. Nos. 4,490,661 and 4,447,771. The brushless permanent magnet motor with sine current wave form is normally classified as an AC synchronous brushless motor, which is mostly used in servo applications where precision is generally required and efficiency of the motor is not important. The sine current wave form drive has more switching losses than those of the brushless DC motor drive. The current wave form of a brushless DC motor is rectangular. The present invention is for a brushless DC motor.
Further, in U.S. Pat. No. 4,834,448 the control switches from a start circuit to a run circuit at a pre-selected speed. In U.S. Pat. No. 5,677,605, a phase advancement for constant power operation above base speed is proposed for a brushless DC motor with Hall sensors as commutation means. However, closed loop speed control for speeds above base speed is not provided.
The unique approach of the present invention is to forward rotate the commutation truth table one or two steps when the motor is required to operate at above base speed. A PI [proportional integration] speed regulator is utilized to generate a signal to determine the time gap between the edge of the most recently triggered Hall sensor and the time to switch to the next step of the above base speed commutation truth table.
According to one aspect of the present invention a method of operating a brushless permanent magnet DC motor having a plurality of Hall sensors for commutation of the motor, a base speed, and an applicable commutation truth table, using at least one jump table based upon the applicable commutation truth table, is provided. The method comprises: (a) Up to base speed operation, effecting commutation of the motor using the applicable commutation truth table triggered by the Hall sensors, with pulse width modulation switching. (b) Above base speed operation, automatically determining a proper time delay between the edge of the most recently triggered Hall sensor and the time to switch to the next commutation step. And (c) automatically regulating motor speed above base speed, utilizing the time delay from (b), and without pulse width modulation switching, by using the at least one jump table.
Preferably (c) is practiced using a PI regulator, and adjusting the proportional gain and integral gain of the PI regulator. Typically, the time delay determined in (b) is TD, and the PI regulator outputs a variable PIOHP, and one sixth of a Hall sensor period is Thall; under these circumstances (b) is practiced to determine TD pursuant to the following equations: when PIOHP is less than AA, TD=1.5*Thall*(AA-PIOHP)/FF; and when PIOHP is equal to or greater than AA, TD=1.5*Thall*[AA/2+FF-PIOH]/FF, AA is equal to 2/3 of FF and FF is the maximum number which can be stored in a signal data memory of a microprocessor. For an 8 bit process, AA=$AA and FF=$FF. Typically, the at least one jump table comprises first or second jump tables, the first jump table comprising the applicable commutation truth table forward rotated one step (60 electrical degrees), and the second jump table comprising the applicable commutation truth table forward rotated two steps; and wherein the PI regulator outputs a variable PIOHP; and wherein (c) is practiced by using the first jump table when PIOHP is less than $AA, and the second jump table when PIOHP is substantially equal to or greater than $AA.
Also, (b) and (c) are practiced so that the time delay for the first jump table gives a synchronous torque angle for about 0-60 degrees, and so that the time delay for the second jump table gives a substantially synchronous torque angle for about 60-85 degrees, and so that the maximum torque angle is limited to about 85 degrees. In the preferred embodiment (b) and (c) are practiced using an at least eight bit microprocessor, and wherein the tables are provided in software in the microprocessor.
According to another aspect of the present invention a controller assembly for a permanent magnet brushless DC motor having a plurality of phases is provided. The controller assembly comprises the following components comprising: A plurality of Hall sensors comprising commutation position sensors for the permanent magnet brushless DC motor. A power amplifier comprising a plurality of power electronic switches connected to the motor, two power electronic switches connected to each phase of the motor. An at least 8-bit microprocessor connected to and controlling each of the power electronic switches. A frequency to voltage converter. The Hall sensors connected substantially directly to the microprocessor, and also connected to the frequency to voltage converter, which in turn is connected to the microprocessor. And wherein the microprocessor, in response to sensing by the Hall sensors both directly and through the frequency to voltage converter, controls the electronic switches to smoothly speed regulate the motor both below and above base speed.
The controller assembly also preferably further comprises a current sensor connected to the microprocessor, and the microprocessor preferably comprises a torque regulator for below base speed operation, and speed regulator and time delay for above based speed operation.
Preferably the electronic switches associated with each phase comprise a top switch and a bottom switch with pulse width modulation (PWM) below base speed, and the microprocessor further comprises a commutation truth table for below base speed operation which, for a three phase motor, comprises:
______________________________________ Hall C Hall B Hall A Gating ______________________________________ 0 0 1 A top on and B bottom PWM 0 1 0 B top on and C bottom PWM 0 1 1 A top on and C bottom PWM 1 0 0 C top on and A bottom PWM 1 0 1 C top on and B bottom PWM 1 1 0 B top on and A bottom PWM ______________________________________
The microprocessor preferably further comprises, for substantially equal to and above base speed operation, first and second jump tables, the first jump table comprising the commutation truth table forward rotated one step, and the second jump table comprising the commutation truth table forward rotated two steps. The first jump table controls the time delay to provide a substantially synchronous torque for about 0-60 degrees, and the second jump table controls the time delay to provide a substantially synchronous torque for about 60-85 degrees.
In the preferred embodiment the motor comprises a three phase motor, and the plurality of Hall sensor comprise three Hall sensors.
According to another aspect of the present invention a method of controlling a permanent magnet brushless DC motor, having a controller as described above, is provided, the method comprises: (a) in response to sensing by the Hall sensors both directly and through the frequency to voltage converter, using the microprocessor to control the electronic switches to smoothly speed regulate the motor both below and above base speed. Also, preferably (a) is further practiced by sensing the current, and by below base speed operating the microprocessor, using the sensed current, to control the torque of the motor. Still further, (a) is practiced by below base speed pulse width modulating the bottom switches with a power amplifier, and using the commutation truth table which, for a three-phase motor, is as described above. Also, (a) is further practiced for substantially equal to and above base speed operation by the microprocessor using first and second jump tables, the first jump table comprising the commutation truth table forward rotated one step, and the second jump table comprising the commutation truth table forward rotated two steps.
It is the primary object of the present invention to provide a brushless permanent magnet DC motor controller, and method of operation of the DC motor, which closes the motor speed loop within the motors entire speed range (from zero speed to speeds far above base speed) without significantly increasing the cost and size of the motor. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.